Cords used on telephone instruments to connect a handset to a base must have sufficient retractility to ensure that they will return promptly to their normal retracted form after having been extended and then released. However, such cords which are commonly known as retractile or spring cords must not be so strongly retractile that they require an excessive amount of force to extend them. If a retractile cord is too unyielding, instead of the cord extending when a pull is exerted thereon, the instrument to which it is attached may be moved on or pulled from its support. Readily extensible retractile cords are desirable, particularly when the retractile cords are connected to lightweight desk-type or bedroom-type telephone handsets. Further, it is economically desirable to obtain an extended length with as short a length of cordage as possible. From an appearance standpoint, it also is desirable that the retracted length of the retractile cord be as short as possible.
Suitable retractility is especially important for cords used on wall mounted telephones. Should the cord not have sufficient retractility, it will sag in an unsightly manner.
Retractile cords of the type used on telephone instruments are generally constructed of highly flexible cordage having a plurality of individually insulated, mandrelated tinsel conductors. Each of these tinsel conductors is made by wrapping a plurality of thin tinsel ribbons of a Phosphor-bronze material, for example, spirally around a multi-filament nylon center core. The tinsel conductor is suitably insulated and, subsequently, the plurality of individually insulated conductors may be jacketed with a plasticized polyvinyl chloride (PVC) composition. The wound cordage is subjected to a heat-treating temperature after which it is removed from a mandrel while the helical direction of the wind is reversed. This construction permits repetitive flexure of the cordage for a relatively large number of times as encountered during normal usage and also permits the cordage to be wound helically during the formation of the retractile cords.
A modular concept in telephone cordage design includes the replacement of individual spade-tipped conductors with a modular plug. Jacks adapted to receive the plugs are mounted in a telephone housing or base and in a wall terminal thereby permitting easy replacement of either the line or retractile cord by a customer or an installer. See, for example, U.S. Pat. Nos. 3,699,498 and 3,761,869 issued Oct. 17, 1972 and Sept. 25, 1973, respectively in the names of E. C. Hardesty, C. L. Krumreich, A. E. Mulbarger, Jr. and S. W. Walden, and U.S. Pat. No. 3,860,316 issued Jan. 14, 1975 in the name of E. C. Hardesty.
Conversion to modularity and its associated plug-terminated cordage necessitated the development of telephone cordage having a smaller cross-section than that used in the past. A cordage design suitable for use with the modular plugs incorporated smaller cross section conductors arranged in a parallel relationship, positioned in a single plane, and encapsulated with a somewhat oval-shaped jacket. In addition to reduced installation costs, the modular cord design offered other potential benefits such as, for example, (1) smaller, lighter weight telephone cords requiring less PVC; (2) in-plant mechanization of cord finishing thereby eliminating manual operations; and (3) replacement of the various color-coded conductors with a single color made possible by the single plane parallel arrangement of conductors for easy identification.
As mentioned hereinabove, the dimensional constraints imposed by the modular plugs and jacks necessitated a reduction in the overall size of both the insulated conductors and jacketed, oval-shaped flat cordage. To reduce the size of the insulated conductor, it became necessary to eliminate a priorly used knitted nylon covering over the served tinsel. The elimination of the protective nylon knit made it necessary to develop a tough insulation material which would function as a high strength barrier to the cutting action of the tinsel ribbons, as an electrical insulation over the tinsel conductor, and as the primary component to achieve resiliency in a retractile telephone cord. A plasticized nylon insulation replaced the knitted nylon covering over the served tinsel conductor and the outer PVC insulation material over the knit.
Subsequently, the individual conductor insulation was changed to include a polyether polyester copolymer composition obtained by reacting 1,4 butane diol terephthalate with terephthalate esters of polyetetramethylene glycol (PTMEG). Such an insulation composition is available commercially from the E. I. DuPont de Nemours Co. under the designation HYTREL.RTM. plastic material. It causes the cordage to have excellent retractility, but is relatively expensive. The use of such an insulation material for cordage is disclosed in U.S. Pat. No. 4,090,763 which issued on May 23, 1978 in the names of W. I. Congdon, et al. and which is incorporated by reference hereinto. The insulation composition also may include a color concentrate comprising a second polyester copolymer which unexpectedly functions as a processing aid when extrusion-coating the composition about the tinsel conductor.
Typically, a relatively flexible PVC is used to jacket the cordage comprising a plurality of insulated conductors. Flexible PVC is made using a range of types and amounts of plasticizers. These materials soften the normally rigid PVC and impart the desired degree of flexibility. However, the plasticizers rarely are very soluble in PVC, and they tend to migrate out of the base material and enter the environment. Migration is a problem from a cosmetic standpoint because the commonly used plasticizers absorb stains during use and migrate back into the surface of the plastic along with the staining substance where they cannot be removed conveniently but where nevertheless they are visible.
In telephone applications, the cords typically have high visibility coupled with high exposure to wear, staining and environmental degradation. Black cords suffer little from staining and only moderately from degradation. However, the increasing demand for cords that are coordinated in color with appliances or interior decor places stringent demands on the PVC jacketing. Staining and discoloration are significant problems, especially with equipment that receives heavy use and has a long service life.
The problems of plasticizer migration and staining are overcome by coating the clear or colored PVC jacket with a barrier layer to prevent interaction between the plasticizer and a potential staining substance. The barrier layer may be a coating of a polyester blend that itself is clear as applied to the cord, and which adheres well to plasticized PVC, is abrasion resistant, flexible, has long term stability against heat and light, can be processed by conventional extrusion, and is itself resistant to stains and discoloration. Such a barrier or top coating, as it is called, is disclosed in U.S. Pat. No. 4,166,881 which issued on Sept. 4, 1979 in the names of W. I. Congdon, et al. The barrier layer, which is relatively expensive, also is effective to provide enhanced retractility for the cordage.
With regard to the coloring of the cordage jacket, more is required than simply to add a standard color constituent. Cords must be provided with jackets which are closely color matched with the colors of the telephone sets to which they are to be connected. To provide a PVC composition which is to be used as the cord jacket with a suitable coloring constituent becomes expensive.
Also, telephone cords are made in varying lengths for varying uses. A typical desk phone, for example, includes a retractile cord which when extended has a length of six feet. A twelve foot retractile cord may be used for wall telephones or for desk phones. For wall telephones, particularly those used in kitchens, cordage having an extended length of twenty-five feet is commonplace.
For the six foot cords, retractility has been achieved and maintained with the conductors being insulated with a thermoplastic elastomeric insulation which is available from the Shell Chemical Co. under the trade designation ELEXAR.RTM. 8481. Because of the relatively short length, a top coating which provides enhanced retractility need not be added to the outer surface of the jacket of the six foot cord.
As one can imagine, as the length of the cord increases, the retractility of the cordage is more difficult to maintain, particularly over time. Twelve foot cords must include provisions for enhanced retractility. This has been accomplished by insulating the conductors of cordage to be used to make twelve foot cords with HYTREL.RTM. plastic material.
For the twenty-five foot cord, particularly one which forms a catenary from the base and handset of a wall-mounted telephone, particular attention must be given to the retractile properties of the cordage. For such a relatively long length, the weight of the cord causes sagging. In this instance, the cordage includes conductors insulated with the ELEXAR.RTM. plastic material and a top coating applied over the cordage jacket. Because the top coating material adds so much to its retractility, the cordage for twenty-five foot cords is tapered with convolutions at each end having a diameter less than the diameter of those in the middle. Tapering the cord helps to control the extensibility and the retractility of the cord. Such a tapered cord structure is disclosed in U.S. Pat. No. 4,375,012 which issued on Feb. 22, 1983 in the names of E. R. Cocco, et al.
As a result of efforts to meet cost and performance requirements, three different length cords comprising three different designs must be inventoried. Because the top coating material is relatively expensive, it has not been used on the six and twelve foot cords. Instead, the six foot retractility requirements have been met with ELEXAR.RTM. plastic material as the insulation for the conductors and the twelve foot cord requirements met with the more expensive HYTREL.RTM. conductor insulation.
As should be apparent, the above requirements and the means for meeting these jacketing requirements have led to costs which are higher than if a universally jacketed cordage were available for all lengths. One could be led to suggest cordage having a top coating be used for any length cordage. However, the top coating material is relatively expensive and its universal use as a portion of the jacket system could affect the cost competitiveness of domestic-produced cordage with respect to those produced out of the country.
What is needed and seemingly what is not available is a cordage structure which may be used for any customary length cord but one which will not adversely affect the cost of any such length cord. Hopefully, such a sought-after cordage will comprise materials currently available in the marketplace and involve manufacturing processes and apparatus which are relatively easy to implement.